The diagnosis and treatment of cardiac disease requires the acquisition of large amounts of data about the patient's medical condition. This data typically consists of medical images, patient information and clinical reports. The treatments of a single patient's disease can generate 1,000,000,000 bytes (1 Gbyte) or more of digital information. Millions of patients are treated worldwide each year for cardiac disease. Many other medical conditions exist for which similar quantities of data are maintained. The storage and management of such patient data requires more capacity than can be cost effectively deployed using standard magnetic disk technology.
The standard industry solution to this problem has been to combine small amounts of hard disk storage with large amounts of low cost magnetic tape storage. This combination of storage technology is implemented as a storage hierarchy. The premise of such a system is that newer data is accessed more frequently than old data. The typical Hierarchical Storage Management (HSM) system software used to manage a storage hierarchy moves data from the more costly hard disk storage to tape storage based on heuristics such as age of the data or last access time. HSM supports this type of data flow transparently to the software application that generates the data. In other words, if the data were a word processed document called mydoc.doc, access to a document is achieved by opening the file mydoc.doc. The HSM system provides access to data by restoring the data from tape to disk automatically. This transparency is created by a virtual file system (VFS). The VFS concatenates all of the storage in the hierarchy into one logical disk drive. The only apparent difference to the user is speed of access; files on the hard disk are accessed very quickly while files on the tape library are accessed with lengthy delays.
The transparency created by HSM creates problems due to monolithic access, proprietary formats, replication difficulties, and poor performance which make HSM poorly suited to mission critical medical image storage environments. Specifically, the primary design principle of HSM is to create transparency so that existing software does not need to be rewritten to take advantage of the large storage system. The transparency is achieved by aggregating all individual storage units (single tapes in a robotic library system plus hard disk(s)) into a single file system. As a result, files will be spread across all of the storage units in the hierarchy.
This design creates a situation where an individual storage unit is only meaningful in the context of the HSM system, which created it. Therefore, if a tape is removed from an HSM managed library, the tape will have no meaning outside of this HSM managed library and the tape must be restored to the HSM system to be accessed. HSM manufacturers often use proprietary or limited logical formats on each storage unit so that tapes cannot be read without the HSM software.
Although HSM often has a system for routine backup, HSM is viewed as a separate function from the backup and recovery management needed in the event of a catastrophe. As a result, HSM systems fail to provide for the creation of offsite copies of tapes for efficiently recreating the HSM in the event of a disaster. The complex data flow in HSM that moves files back and forth between levels in the hierarchy results in poor performance. An application must wait for a file to be fully restored from tape to disk before accessing the first byte of information.